Engineering the Roman Colosseum: Architecture and Design

Introduction

"While the Colosseum stands, Rome shall stand; when the Colosseum falls, Rome shall fall; when Rome falls, the world shall fall." — The Venerable Bede, quoting an ancient Anglo-Saxon peasant prophecy

Perhaps the most enduring symbol of the greatness of the Roman Empire can be seen today in the ruins of the Colosseum. This massive amphitheater is situated in the middle of modern Rome near the Roman Forum and has become an iconic representation of the Roman Empire at its zenith. Although estimates vary, analysts believe that at least 50,000 and perhaps as many as 80,000 spectators were accommodated within its capacious dimensions, and the Colosseum has become the benchmark by which all subsequent stadia have been judged. Flush with the treasures and riches of Jerusalem, the builders of the Colosseum spared no expense in its design and construction. Despite its impressive seating capacity and functional design, opinions concerning the aesthetics of the Colosseum have varied over the centuries, with some scholars describing the structure in strictly superlative terms while others have argued that it lacks the aesthetic qualities that characterize other Roman architecture. This paper reviews the relevant literature to provide the dimensions of the Colosseum, important architectural details, and varying scholarly views concerning the end result, before summarizing key findings in the conclusion.

Overview and Dimensions of the Colosseum

The location selected for the construction of the Colosseum was a level region situated between the Esquiline, Caelian, and Palatine Hills.² According to one historian, "Constructed in the city center, the Colosseum differed vastly from other amphitheaters that were typically located on the outer edge of a city. It emblematically stood, and still stands, at the core of Rome."³ Irrespective of any aesthetic considerations, the dimensions of the Colosseum are truly impressive even by modern standards. In sum, the structure measured 189 meters (615 feet) long and 156 meters (510 feet) wide; its base area covers six acres, and the height of the outer wall is 48 meters (157 feet), making it as tall as a modern 12–15 story building.⁴

Although significantly smaller today due to the harvesting of stones for local construction projects over the centuries, the Colosseum's perimeter was originally 545 meters (1,788 feet). The central arena's oval measured 287 feet long by 180 feet wide, and this enclosure was surrounded by a 15-foot high wall.⁵ The various events held at the Colosseum — including gladiatorial contests known as munera — were sponsored by private Roman citizens rather than the Roman state. As Barbi reports, "While every program contained a religious component, they were mostly organized to exhibit familial power and social status. In fact, it was obligatory for the ruling class to coordinate the games in order to keep the citizens content, as the munera were extremely well liked among the residents of Rome."⁶

Construction on the Colosseum began between 70 and 72 AD during the reign of Emperor Vespasian, for an explicitly political reason: to ensure the popularity of the emperor, a need that became especially acute following the rule of the widely despised Nero. As Barbi explains, "The structure was to be a present to the Roman people and was to be constructed in the place where the previous Emperor Nero had built his residence, the Domus Aurea."⁷ Some historians also believe that the name "Colosseum" derives from a colossal bronze statue of Emperor Nero that had stood nearby.⁸

At the time of Vespasian's death in 79 AD, construction had progressed to the point where the third level had been completed.⁹ By 80 AD, his son Titus had succeeded in completing the top levels, and the Colosseum was officially dedicated to the people of Rome. As Sear notes, "Vespasian was succeeded by his elder son, Titus, who in his short reign endeared himself to the Roman people as one of the best-loved Roman Emperors. He inaugurated the Colosseum in AD 80."¹⁰

Despite its enormous size, the Colosseum's structure is completely self-supporting — a fact that is all the more impressive given its six-acre foundation.¹¹ The Colosseum's outer wall was originally comprised of more than 3,000 feet of travertine stone, though this has been seriously degraded over the centuries. The north side of the wall remains largely intact, but the current exterior of this wall is actually the original interior wall.¹²

Building Materials and Construction Methods

A number of different types of locally sourced building materials were used to construct the Colosseum, including a type of limestone known as travertino, tuff (used for radial walls and some pillars), ceramic tiles (used on walls and for the flooring of upper levels), and concrete used for the vaults.¹³ Although the historical record is unclear on the point, it appears reasonable to suggest that the floor of the main arena was also made from some type of waterproof concrete. As Grayson notes, "The Romans restaged battles in the Colosseum so that the vulgar could celebrate famous victories in which the players were actually killed and, if it was a naval engagement, the arena was filled with water."¹⁴

There were in fact two different types of mortar used in the Colosseum's construction. One of these, pozzolanic mortar, was waterproof and could account for the ability of Roman engineers to flood the arena for naval battles without inundating the structure's lower levels. This mortar consisted of pozzolana, made from volcanic ash that had been sufficiently heated to produce Portland concrete-type waterproof qualities.¹⁵ In his study "The Riddle of Ancient Roman Concrete," Moore reports that Roman engineers discovered by trial and error that when a small amount of volcanic ash was reduced to a fine powder and mixed with moist lime, it produced a waterproof concrete that would even cure underwater.¹⁶

An important finding from Moore's study concerns the placement method employed by Roman engineers: "We have learned that ancient concrete was a simple mixture of wet lime and pozzolan in specific ratios to match the desires of the Roman architect. We have also learned that the Romans followed a placement method of tamping their stiff mortar into the voids of a rock layer."¹⁷ By tamping the mortar, the Romans were able to eliminate excess water, thereby increasing the strength of the resulting mixture — a process that was only recently rediscovered by modern engineers.¹⁸

The various building materials used in the construction of the upper levels were undoubtedly moved into place using a human-powered treadmill crane. As one historian reports, "Columns and other massive blocks were raised by means of a crane with its system of pulleys. Motive power was supplied by men in a treadmill — a huge wooden wheel."¹⁹

Conclusion

The research showed that construction began on the Colosseum between 70 and 72 AD during the reign of Emperor Vespasian and was completed in 80 AD by his son Titus, using riches plundered from the Roman sack of Jerusalem. Most modern architectural scholars maintain that the Colosseum represents the epitome of Roman engineering, but some earlier scholars argued that the structure was actually poorly designed and not particularly aesthetically pleasing. Notwithstanding these isolated criticisms, there is a general consensus today that the Colosseum was a masterpiece of Roman engineering that deserves to be protected and studied further — especially given the Romans' early mastery of concrete, which allowed them to construct massive structures with the waterproof qualities needed to flood the main arena for reenacted naval battles. The Colosseum's legacy as a model for crowd management, structural engineering, and monumental public architecture remains undiminished nearly two millennia after its completion.

Appendix A: Dimensions of the Colosseum

Shape: Oval

Capacity: Maximum 80,000 spectators

Area covered: 6 acres

Number of entrances: 76 numbered public entrances plus 4 unnumbered grand entrances for elites

Height: 50 meters (187 feet), equivalent to a 12-story building

Length: 189 meters (615 feet)

Width: 156 meters (510 feet)

Outer circumference: 545 meters (1,788 feet)

Center arena dimensions: 287 feet long and 180 feet wide, covered with 15 cm of sand

Arena wall height: 15 feet

Vaults: Spanning between 80 radial walls to support tiers of seating, passageways, and stairs

The Hypogeum: Two-level subterranean network of tunnels and 32 animal pens; 80 vertical shafts providing instant arena access for animals and scenery; large hinged platforms called hegmata for large animals

Trap doors in arena: 36

Retractable awning (the Velarium): Three stone brackets in each of the 80 bays; brackets supported 240 wooden masts on which canvas awnings were hung

Drains: Built 8 meters (26 feet) underneath the structure

Foundations (outer walls and seating): 12–13 meters (39–42 feet) deep

Foundations (inner ellipse of arena): 4 meters (13 feet) deep

Seating allocation per spectator: 40 cm width; 70 cm leg room

Marble used: Approximately 100,000 cubic meters

Rows of seats: 60–80 rows

Level 1 (Ground level): 34 feet high; Doric arches 23 feet high and 14 feet wide

Level 2: 38 feet high (Ionic order); arches 21 feet high and 14 feet wide

Level 3: 37 feet high (Corinthian style); arches 21 feet high and 14 feet wide

Level 4: 45 feet high; no arches